Dynamic tweening is really important to a Flash Developer, and when I was programming in AS2 my tweening library of choice was either FuseKit or the Zigo Engine. Since AS3 there have been a few options available:

# Tweener
# Tween Lite
# Adobe's Tween
# GTween
# ASAP Tween

There have been many posts on how some are better in performance and features than others. Without going into detail, I rate them in the following order, from best to worst:
1) TweenLite/Max
2) Tweener
3) GTween
4) ASAP Tween
5) Adobe Tween

Even though there are 5 options above, the top 2 are the only ones I'd ever consider using. Even with this said, Tween Lite is good, but it's not a code library I'd prefer to use. If anyone cares to take a look through the source code, they'll notice that it's not particularly clearly (well) written. Because of this, it's not exactly easy for a developer to extend Tween Lite/Max if they need to.